1. Field Of The Invention
The present invention pertains generally to atomic absorption and plasma spectrometers, and more particularly, to spectroanalytical systems of the type that sense a special characteristic of a flame by absorption or emission technology. The present invention more specifically pertains to a system to optimize the spectroanalytical analysis of a given sample by modifying the physical characteristics of the sample spray routed to the burner or plasma torch.
2. Description of the Prior Art
In atomic absorption and plasma spectroscopy, measurement of the absorption or emission of radiation at a characteristic resonant spectral line for a particular element yields a measure of the concentration of that element in an original sample. Presently, the most common technique for atomizing an element for purposes of absorption measurement is by introducing a liquid sample containing the element of interest into a gas burner wherein droplets of the sample are vaporized and the elements ultimately atomized so as to form a radiation beam. In plasma emission systems, a liquid sample is nebulized with a plasma gas such as argon, nitrogen, etc. The sample liquid is suspended in microsized droplets which are introduced into the plasma torch wherein the atoms of the liquid are energized from the plasma. Energy emitted by the energized sample is measured by the spectroanalytical system.
In atomic absorption or emission type spectroanalytical systems, the material to be analyzed is introduced into a premix or gas flow chamber by a nebulizing arrangement using a regulated plasma gas or oxidant stream. The plasma gas or oxidant stream is ideally introduced into the gas flow chamber as a fine uniform spray of minute droplets, which droplets are entrained with a combustible fuel or plasma gas and flow through the gas flow chamber into a burner or plasma torch. Upon combustion, the combustible fuel energizes the material to be analyzed for purposes of analysis. In plasma units, the plasma torch energizes the material. An example of a nebulizer arrangement is seen in U.S. Pat. No. 4,220,413.
A nebulizer generally employs a Venturi-type restriction which passes rapidly-moving plasma or gas past an opening, thereby drawing a portion of a liquid sample into the gas stream thereby effecting an atomizing of the liquid. The liquid is aspirated by the Venturi effect caused by the rapidly moving current of gas. In an absorption system, sample laden gas or oxidant then passes into the gas flow or premix chamber, where it is mixed with additional oxidant from an auxiliary inlet and a fuel such as acetylene. The mixture is then introduced into the burner head where it is ignited. Oxidants and fuels are not introduced in plasma systems. Instead, in such systems, the sample laden gas is introduced directly into the plasma torch via a screen type nebulizer or the like.
The sensitivity of an absorption measurement is dependent on a number of factors. In an absorption system, one such factor is the flame condition of the burner--i.e. the leanness or richness of the fuel oxidant mixture. Also, the sensitivity of the measurement usually requires optimization of the setting of the nebulizer which varies the amount of liquid sample aspirated to the burner or plasma torch. Because of the nature of the mechanism for aspirating more or less of the sample, namely varying the flow of oxidant or plasma gas through the venturi-type restriction, there is the obvious side effect on the flame condition or torch which has a direct effect on the sensitivity of the measurement.
In an attempt to achieve a consistent flame or plasma condition, a number of premix or gas flow chambers have been developed. The most common design comprises a first tube horizontally disposed in a larger second tube so as to define a ring-like annulus therebetween. The smaller tube is situated in fluid communication with a nebulizer arrangement at one end and is of a length sufficient to allow a flow space between the remote end of the smaller tube and the larger tube. A burner head or plasma torch is coupled to the larger tube via a passageway disposed in the upper portion of the tube. Sample laden gas introduced into the smaller tube "paints" the inside of the smaller tube and the end of the larger tube, whereupon the sample laden gas then doubles on itself wherein a portion of the combination is moved into the passageway leading to the plasma torch or burner. Due to this characteristic flow path, this type of spray chamber is commonly called a "double pass" spray chamber.
Double pass and similar spray chambers present a number of disadvantages. The design of the double pass gas flow system was motivated by a desire to substantially reduce the introduction of large sample droplets into a burner (or plasma torch in the case of plasma spectroscopy). Therefore, such designs require that a significant surface area of the spray chamber be "painted" by the mist of a liquid sample preliminary to the introduction of the sample into the plasma torch or burner. In this connection, a conventional embodiment of the double pass chamber typically compels spray droplets to travel some nine inches prior to introduction into the burner or ICP plasma torch.
As a result of such designs, however, a relatively low concentration of a given sample solution is actually introduced into the burner or torch. The result of this low concentration flow is the accumulation of solid deposits in the passageway (injector tube) leading to an ICP torch (or burner in the case of atomic spectrometers). This deposition of solid particles is particularly pronounced in plasma spectroscopy, where the ICP torch operates at temperatures in the order of 8000.degree. K. When a low concentration of droplets enter the injector tube (such as in the case in the double pass system), the sample droplets are prone to rapid evaporation. Such evaporation results in the deposition of salts in this tube. These salts must be periodically removed from the injector tube to allow for continual and reliable burner operation. In the case of conventional double pass systems, for example, these salts must be physically removed after every hour of operation where liquid samples having a high salt content are employed.
Aside from the inconvenience compelled by the necessary maintenance of injector tubes, the presence of salts is also undesirable from the standpoint of increasing the coefficient of variation for sample replication. Further, the continual deposition of salts in an injector tube also reduces long term instrument stability.
Other disadvantages of prior art spray chambers include the uneven distribution of sample spray droplets inside the burner or torch. Uneven spray distribution is brought about as a result of large droplets. This problem is especially pronounced when measuring small concentrations of a sample of interest. A large sample droplet, when conveyed to the torch or flame, can result in erratic analyses. The overall result of uneven spray distribution is a loss of instrument sensitivity.